1. Field of the Invention
The present invention relates to a multilevel inverter that converts direct current power to alternating current power.
2. Description of the Background Art
In relation to an inverter that converts direct current power to alternating current power, the spread of a multilevel inverter wherein connections to a positive terminal, negative terminal, and intermediate terminal to which direct current voltage is applied are switched by switching elements (semiconductor switches) is increasing. As the switching voltage in the switching element connected to the intermediate terminal is low compared with that in the switching elements connected to the positive terminal and negative terminal, it is possible to select a switching element with low rated voltage, and thus possible to employ a switching element with comparatively good electrical characteristics.
However, in the event that all the switching elements are simultaneously turned off when attempting an emergency stop of a motor connected to the inverter, the difference in potential between the positive terminal and negative terminal is applied to the switching element with low rated voltage connected to the intermediate terminal, and there is concern about breakage. Therefore, there has been proposed a configuration wherein, when carrying out an emergency stop of a motor, the switching elements connected to the positive terminal and negative terminal are turned off first, and the switching element connected to the intermediate terminal is turned off after a slight delay (for example, refer to JP-A-2002-078351).
In recent years, however, in order to avoid personal damage from accidents caused by a breakdown of an electrical instrument, or the like, various international standards, established in order to bring risk within a tolerable range, have been developed. For example, IEC61800-5-2 requires the inclusion of a safe torque off (STO) function that, when carrying out an emergency stop of a motor, reliably stops the motor.
In relation to a two-level inverter, a configuration wherein a circuit that disables a switching element connected to a positive terminal and a circuit that disables a switching element connected to a negative terminal are provided individually has been proposed as a configuration appropriate for IEC61800-5-2 (for example, refer to JP-A-2010-284051). A two-level inverter is such that output to the motor is interrupted provided that the switching element of one of the positive terminal or negative terminal is disabled, because of which, even in the event that one of the heretofore described circuits breaks down, the motor is stopped provided that the other circuit functions normally.